EP0226140A2 - Method and apparatus for the combustion of solid fuels in a circulating fluidized bed - Google Patents

Abstract

In a method for the combustion of solid fuels in a fluidised-bed reactor (1) with a circulating fluidised bed, the combustion gas discharge being influenced by the supply of air in a number of stages and at different heights of the fluidised-bed reactor, and ash removed from the fluidised-bed reactor being separated and conducted back into the fluidised-bed reactor, optimum combustion conditions over a wide load range, in particular for outputs below 50 MWth, are achieved simply in that separated ash is conducted back into the fluidised-bed reactor at a temperature in the range between 20 and 250 DEG C and at the same time the quantity of the ash flow conducted back is regulated depending on the combustion chamber temperature. This is preferably carried out by an uncooled combustion chamber (2), a flue gas cooler (6; 7), which is flowed through vertically, has cooling surfaces (8, 9, 10, 11) and diverts the flue gas at least once, and a filtering dust separator (14), which are arranged one behind another, and an ash return line (24) leading from the dust separator (14) to the reactor chamber (2). <IMAGE>

Description

The invention relates to a method for burning solid fuels in a fluidized bed reactor with a circulating fluidized bed, the combustion process being influenced by supplying oxygen-containing gas, in particular air, in at least two stages and at different heights of the fluidized bed reactor, combustion heat being dissipated via cooling surfaces, combustion products , containing ash and exhaust gas, discharged from the fluidized bed reactor, ash separated from the combustion products and subsequently at least partially returned to the fluidized bed reactor. The invention further relates to a device for performing this method.

There are currently two basic methods for burning carbon-containing materials in suspension while removing combustion heat through cooling surfaces: (1) Fluidized-bed firing with a "stationary" fluidized bed, in which a comparatively dense "bed" forming the fluidized bed by a clear density jump from the overlying one , practically solid-free gas space is separated, and (2) working with a "circulating" fluidized bed, in which the solid discharged from the fluidized bed reactor is separated from the flue gas in a separator and is completely returned to the fluidized bed reactor via a return line, while excess solid is drawn off . With the stationary fluidized bed, today's requirements for keeping the air clean can no longer be met. Known processes with circulating fluidized beds, which are generally cheaper in terms of keeping the air clean, can only be used in economically justifiable expenditure in the power range above about 50 MW _th .

From the German patent 25 39 546 a method for burning solid fuels in a circulating fluidized bed is known, in which the adaptation to the respective power requirement takes place in that the cooling surfaces at least in Reactor room and optionally also in a downstream fluid bed cooler. In order to influence the heat transfer to the cooling surfaces above a secondary gas supply, an average suspension density of 15-100 kg / m³ is created by adjusting the amount of fluidization and secondary gas and the heat of combustion is dissipated predominantly by means of the cooling surfaces located above the secondary gas supply within the free reactor space. This procedure is intended to achieve (a) a temperature constancy which is characteristic of circulating fluidized beds in the entire circulation system, (b) uniform combustion with little nitrogen oxide evolution and (c) rapid and good distribution of the carbon-containing material introduced. The adaptation to fluctuating power requirements takes place by regulating the combustion output via the suspension density in the upper reactor section, specifically by changing the amounts of fluidization and secondary gas.

A large-scale application of the circulating fluidized bed as a means of burning solid fuels has so far not been possible for the power range below about 50 MW _th , since the required handling of hot ashes to be recirculated appeared too complex, a relatively complex combustion chamber cooling is necessary for small plants and proven simple heating surface systems, such as large water room or steep tube boilers or similar could not be used; after all, the control effort and the demands on the operating personnel were also relatively high.

Proceeding from this, the invention has for its object to provide the method of the type mentioned and an apparatus for performing this method, which do not have the aforementioned disadvantages and with which optimal combustion conditions in an extended performance range, in particular below about 50 MW _th with economically justifiable effort.

In procedural terms, this object is achieved in that at least a portion of the separated ash is returned to the fluidized bed reactor at a temperature in the range between 20 and 250 ° C., the temperature in the fluidized bed reactor is measured at at least one point and the at least one recycled ash portion is dependent is controlled by the temperature measured in the fluidized bed reactor. The device according to the invention for performing this method is defined in claim 11.

The invention eliminates the hot gas cyclones and furnace bores necessary in known processes, and on the one hand the fluidized bed coolers specially used to cool the ashes are superfluous, and on the other hand, proven and cheap shell boilers can be used even in the low power range below 50 MW _th . A large water chamber flue tube boiler or a water tube boiler can be installed directly downstream of the actual fluidized bed reactor, for example directly on top.

The majority of the returned ash is obtained from a dust separation system downstream of the cooling surfaces, i.e. in the cold part of the plant. The amount of ash recovered and recycled from the dust separators can be easily regulated so that the optimum temperatures in the fluidized bed for combustion and pollutant reduction are maintained. An essential prerequisite for problem-free regulation of the optimal temperatures by returning cold ashes is that the fire or reactor space remains completely or at least predominantly uncooled.

In order to be able to more easily adjust the optimal combustion conditions over a large load range, in particular below 50 Mw _th and when using solid fuels in a wide quality range, including ballast fine coal and waste fuels, a further development of the invention provides that the separated ash is temporarily stored and one of the Amount of ash dependent on fuel quality is stored out of the intermediate store and returned to the fluidized bed reactor.

The part of the ash returned from the dust separation system has a temperature of less than 250 ° C when it emerges from the dust separation system. The remaining part of the returned ash can be at a higher temperature, in extreme cases even at a fluidized bed temperature, if it is generated in a hot part of the circuit by deflection. As a result of multiple deflections and separation in the cold part of the system, ash with different temperatures or from storage containers serving as temporary storage is available. The proportions and temperatures of the recycled ash can therefore preferably be selected in the context of an automatic quantity control in such a way that the optimum combustion temperatures for the combustion and minimization of pollutants are maintained.

The separated ashes can be returned to the furnace via an open or a closed return line, e.g. also be done pneumatically or by means of a mechanical conveyor.

• Fig. 1 zeigt in schematischer Darstellung eine Verbren­nungsanlage nach der Erfindung.
• Fig. 1A ist eine schematische Schnittansicht entlang der Linie A-A in Fig. 1 durch den Reaktorraum mit Darstellung der Sekundärgaszuführung.
• Fig. 2 zeigt in schematischer Darstellung eine Verbren­nungsanlage ähnlich der jenigen gemäß Fig. 1 mit zusätzlichen Staubabscheidestufen.
• Fig. 3 zeigt eine modifizierte Verbrennungsanlage nach der Erfindung in schematischer Darstellung.

Further details, features and advantages of the invention result from the following description of the accompanying drawing, in which three preferred exemplary embodiments of an incineration plant according to the invention are shown.

• Fig. 1 shows a schematic representation of an incinerator according to the invention.
• FIG. 1A is a schematic sectional view along the line A - A in FIG. 1 through the reactor space, showing the secondary gas supply.
• FIG. 2 shows a schematic illustration of an incineration plant similar to that according to FIG. 1 with additional dust separation stages.
• Fig. 3 shows a modified incinerator according to the invention in a schematic representation.

1 to 3, all components having the same effect are provided with the same reference numbers. In all exemplary embodiments, a fluidized bed reactor is provided which is lined with ceramic material, in particular by lining. The reactor or combustion chamber 2 is completely free of cooling surfaces and has an inflow floor 3 for fluidizing gas and at least one secondary gas line 4 opening into the combustion chamber 2 above the inflow floor 3, and a solids feed device 5. A swirl is generated by the approximately tangential introduction of secondary gas in the direction of arrows 4a in FIG. 1A into the fluidized bed, which sets the gas-solid suspensions into a rotary movement promoting the mixture.

1 and 2, the flue gas leaves the uncooled fluidized-bed chamber 2 essentially without deflection, flows inside a flue gas cooler 6 designed as a large-volume smoke-tube boiler or water-tube boiler, initially vertically upwards, and only gives a first part of its heat outside the fluidized-bed chamber 2 Cooling surfaces 8 from. By means of a downstream pipe bend or the like serving as a flue gas deflector 12. the flue gas is subsequently deflected by 180 ° and then flows vertically downwards again through a second stage of the flue gas cooler 6, whereby it emits a further part of its heat to cooling surfaces 9.

In the simplest embodiment of the incineration plant shown in Fig. 1, the flue gas leaving the flue gas cooler 6 arrives in a falling stream in a dust separator 14 which e.g. is designed as an electrical or bag filter and its flue gas contact surfaces 6 can have a coating of catalytically active material for additional separation of pollutants.

The flue gas cleaned in this way leaves the dust separator 14 via a flue gas line 15, from which it goes directly into the chimney. The solid which is separated in the dust separator 14 and mainly consists of ash collects in at least one separating funnel 16, which is preferably designed as an ash storage container or intermediate store. The ash temporarily stored in the separating funnel 16 has reached a temperature in the range below 250 ° C. which is suitable for entry into the reactor space by cooling in the cooler 6. It is fed through at least one ash separation line 20 attached to the bottom of the separating funnel 16 and a metering device 30 to a conveyor system 24 connected to the solids feed device 5, on which a discharge 25 for excess ash is also provided. All other materials to be fed to the furnace, namely the fuels and possibly additives for reducing pollutants, for example lime, can also be fed into the return line 24. Together, these substances form a so-called extended bed within the combustion chamber 2 of the fluidized bed reactor.

The amount of ash removed from the conveyor system 24 is regulated in the invention as a function of the temperature in the combustion chamber 2. For this purpose, the temperature in the combustion chamber is measured via at least one temperature sensor 26, the temperature measurement value is fed to a controller or processor 27 and with a predetermined one Setpoint compared. The setpoint input is shown schematically by line 28. The setpoint can be entered manually or according to a sequence program. The processor output signal controls the dosing device 30 and thus the returned amount of relatively cold ash via a line 40. Depending on the calorific value, heat load of the incineration plant and temperature of the returned ash, the metering device 30 is set so that preferably 0.5 to 3 kg ash / s per MW of combustion output are returned to the combustion chamber 2.

The described system for controlling the ash return works in principle as follows: If the combustion chamber temperature (26) rises above the setpoint (28) for any reason, the amount of ash flow returned from the dust separation system is increased. This recycled ash, which has already been discharged from the combustion chamber at least once, is so fine that when it is supplied again to the combustion chamber, it leaves it again, whereby it can absorb heat and transport it from the combustion chamber to the cooling surfaces. As a result, the furnace temperature drops.

If the combustion chamber temperature is to be increased, the amount of ash returned is reduced accordingly. A fluctuation in the combustion chamber temperature in the range of 850 ± 70 ° C appears to be harmless for the process. This corresponds to a fluctuation in the returned ash quantity of ± 10%, which does not have to be compensated for by the control.

In contrast to FIG. 1, a flue gas deflector 13 is arranged in the flow direction of the flue gas between the flue gas cooler 6 and the dust separator 14 in the modified combustion system according to FIG 21 has. Ash stored in the hopper 17, metered by a metering device 31, can be discharged through the line 21 into the conveyor system 24 designed as a collecting line. The dust separator 14 shown in FIG. 2 has two separating funnels 16 with associated separating lines 20 which feed partial quantities of ash into the conveyor system 24 via metering devices 30a and 30b. The metering devices 30a and 30b can preferably be controlled by the processor 27 via separate signal paths 40a and 40b. The metering device 31 is controlled via a control line 44. With the additional separating funnels, the volume of the total ash supply available on the cold side of the combustion circuit increases, so that greater fluctuations in the heat load can be compensated for by better metering of the ash quantities and different ash temperatures. In addition, ashes of different grain sizes are available, which can be used as an additional control variable due to their different time in the combustion chamber. The temperature of the ashes drawn off via the lines 20 from the separating funnels 16 is approximately 100 ° C. to 250 ° C., while the temperature of the ashes from the deflection 13 can be higher.

In the ash return line 22 for uncooled, ie hot, ashes, as in the other return lines, a metering device 32 is arranged, which can be controlled by the processor 27 via a line 42 with a control signal for setting the flow rate.

In the embodiment shown in FIG. 3, the flue gas after leaving the fluidized bed reactor 1 and before entering the cooler 7, a deflection 13 ', which is equipped with a separating funnel 18 and an ash return line 22. In the flue gas cooler 7, the flue gas gives off a first part of its heat to cooling surfaces 10 in the initially falling stream. At the lower end of the flue gas cooler 7, a flue gas deflector 13 'is arranged, in which the flue gas is deflected by approximately 180 °, so that it subsequently releases a further part of its sensible heat to cooling surfaces 11 of the flue gas cooler 7 in the now increasing gas flow, whereupon it follows further deflection, flows through a dust separator 14 corresponding to FIG. 2.

A further separating funnel 19 with ash return line 23, metering device 33 and control line 43 is assigned to the flue gas deflection 13˝.

The ash separation lines 20, 22 and 23 open into the (return) conveyor system 24, the temperature of the ash in the separation lines 20 (cold side) approximately 100 ° C. to 250 ° C., in the ash separation line 23 approximately 400 ° C. to 600 ° C. and in the ash separation line 22 is about 700 ° C to 850 ° C. The processor 27 is programmed to control the dosing devices 30-33 for dosed ash removal through the individual ash removal lines to maintain the furnace temperature in a desired target temperature range.

In all of the exemplary embodiments described above, the temperature in the fluidized bed is kept at approximately 850 ° C. ± 70 ° C. Cold ash (in the temperature range between 20 and 250 ° C.) is preferably returned to one of the fires by returning dust separators 14 and deflectors, for example 17, which are always present corresponding output. The fire or reactor chamber 2 is always uncooled, that is, without heat exchange internals.

Claims (17)

1. A method for burning solid fuels in a fluidized bed reactor with a circulating fluidized bed, the combustion process being influenced by supplying oxygen-containing gas, in particular air, in at least two stages and at different heights of the fluidized bed reactor, combustion heat being dissipated via cooling surfaces, containing combustion products Ash and exhaust gas, discharged from the fluidized bed reactor, ash separated from the combustion products and subsequently at least partially returned to the fluidized bed reactor,
characterized by
that at least a portion of the separated ash is returned to the fluidized bed reactor at a temperature in the range between 20 and 250 ° C, the temperature in the fluidized bed reactor is measured at at least one point and the at least one recycled ash portion is regulated as a function of the temperature measured in the fluidized bed reactor . 2. The method according to claim 1, characterized in that the heat generated in the fluidized bed reactor is drawn off outside the combustion chamber in a downstream cooling system from the ash-containing combustion products. 3. The method according to claim 1 or 2, characterized in that at least a portion of the ash removed in the combustion products is separated after a first cooling step by at least one downstream dust separation system. 4. The method according to any one of claims 1 to 3, characterized in that the separated ash is temporarily stored in at least one intermediate store and that at least a partial amount is metered from the intermediate store and in the fluidized bed reactor is returned in order to be able to regulate the combustion chamber temperature under different loads. 5. The method according to any one of claims 1 to 4, characterized in that the portion of the ash to be returned to the fluidized bed reactor with other solid feedstocks, in particular at least one further ash portion, solid fuels, additives for reducing pollutants and / or mixtures of the aforementioned substances , mixed and then introduced into the fluidized bed reactor via a common entry. 6. The method according to any one of claims 1 to 5, characterized in that the combustion products containing ash discharged are deflected at least once along a flow path in the cooling system and that at least one partial ash quantity is separated from the combustion products after the deflection. 7. The method according to any one of claims 1 to 6, characterized in that several ash subsets with different ash temperatures are introduced into the fluidized bed reactor. 8. The method according to any one of claims 1 to 7, characterized in that the at least one recirculated portion of the ash is controlled in dependence on the furnace temperature such that the furnace temperature is kept in a temperature range favorable for the combustion conditions of the fuels. 9. The method according to any one of claims 1 to 8, characterized in that at least one measured value of the furnace temperature entered into processor means and a target range of the furnace temperature is specified in the processor means, that the at least one measured value is compared with the predetermined target range and that at least one control signal for changing the flow rate of the at least one partial ash quantity returned to the furnace is developed if the measured furnace temperature is outside the specified target range in order to reset the furnace temperature to the target range. 10. The method according to claim 9, characterized in that the target range of the furnace temperature is between about 780 and 980 ° C. 11. Device for burning solid fuels in a fluidized bed reactor (1) with a circulating fluidized bed, comprising a reactor chamber (2) with bottom-side means (3) for supplying fluidized gas for the production of the fluidized bed, means (5) for supplying solid fuels and the circulating fluidized bed in the reactor space, means for discharging combustion products from the reactor space along a predetermined flow path, the flow path including at least one deflection, cooling surfaces (8, 9; 10, 11) for removing combustion heat from the combustion products, separating means (14) for separating ash from the combustion products and ash return means (20, 24) which are connected to the reactor space, for returning ash to the reactor space,
characterized by
that an uncooled reactor chamber (2), at least one preferably vertically flowed, with cooling surfaces (8, 9; 10, 11) provided flue gas cooler (6; 7) and at least one filtering dust separator (14) are arranged one behind the other in the flow direction of the combustion products, that the Dust collector (14) at least one store (16, 19) for temporarily storing the separated ash is assigned to the mass flow metering means (30a, 30b, 33) for metering ash removed from the at least one store (16, 19) with an ash return line (20, 21, 22, 23) that at least one temperature measuring device (26) for measuring the fluidized bed temperature at at least one point in the reactor chamber (2) and control means (27, 40a, 40b, 41; 42, 43) for regulating the temperature in the combustion chamber recycled amount of ash as a function of the measured furnace temperature are provided, the control means being coupled to the measuring device (26) and the mass flow metering means. 12. The apparatus according to claim 11, characterized in that the reactor chamber (2) is lined with ceramic material. 13. The apparatus according to claim 11 or 12, characterized in that the flue gas cooler (6; 7) is designed as a large water space smoke tube boiler or as a water tube boiler. 14. Device according to one of claims 11 to 13, characterized in that a vertically flowed flue gas cooler (6) is connected to the uncooled reactor chamber (2), which has at least one U-shaped flue gas deflector. 15. The apparatus according to claim 14, characterized in that between the reactor chamber (2) and the dust separator (14) at least one deflection (13; 13 '; 13˝) is arranged and that further ashes of higher temperature are branched off and in from the existing deflections the reactor space (2) is returned via ash lines (21, 22, 23). 16. The device according to one of claims 13 to 15, characterized characterized in that the flue gas deflector (13 ') has a lower section (18), from which an ash separating line (22) for hot ash serving as a return line branches off. 17. Device according to one of claims 11 to 16, characterized in that at least the dust separator (14) has a coating of catalytically active material.

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